单项选择题
An（）indicates that there is imminent risk for navigation.
A.distress alert B.urgency message C.safety message D.routine message
单项选择题 A(n)（）indicates that there is serious danger for vessel, crew and passenger.
单项选择题 A(n)（）indicates that is serious and inmmediate danger for vessel, crew and passengers.
单项选择题 Cirrocumulus belongs to（）.
